In recent years, in the field of magnetic recording, digital recording which is less in deterioration of recording is developing towards practical use in place of conventional analog recording. Recording and reproducing units and magnetic recording media which are used for the digital recording are required to have high image quality and high tone quality, and needs for realizing size reduction and space saving are increasing. However, in general, the digital recording is required to achieve recording with a number of signals as compared with the analog recording. For that reason, magnetic recording media which are used in the digital recording are required to achieve recording with still more high density.
In order to achieve the foregoing high-density recording, it is essential to make the wavelength of a recording signal short and to make the recording track narrow. Accordingly, in addition to the technology for granulating and highly packing a ferromagnetic power and the technology for smoothing the surface of a magnetic recording medium, there has hitherto been developed the technology for thinning a magnetic recording medium for the purpose of enhancing a volume density.
As the foregoing technology for thinning a magnetic recording medium, there have been known so far a method for thinning a non-magnetic support and a method for thinning a non-magnetic layer. However, the method for thinning a non-magnetic support involved a problem that when the thickness becomes smaller than a prescribed value, durability of the non-magnetic support is lowered.
On the other hand, the method for thinning a non-magnetic layer involved problems, for example, a lowering of output, an increase of dropout, and an increase of error rate, because the magnetic layer is likely influenced by the surface of the non-magnetic support. That is, if thinning of the magnetic recording medium for the purpose of increasing the recording density is advanced, there was encountered a problem that a sufficient leveling effect against the non-magnetic support is not obtained in the magnetic layer, the surface state of the non-magnetic support as provided beneath the magnetic layer or non-magnetic layer largely influences the surface of the magnetic layer so that projections are formed on the surface of the magnetic layer, thereby causing an error rate, etc. In particular, in a linear recording system, since a magnetic tape runs substantially parallel to a head and comes into contact with the magnetic head, dropout due to projections present on the surface of the magnetic layer is likely generated.
In order to solve the foregoing problems involved in thinning a magnetic recording medium, it was necessary to suppress influences of the surface of the non-magnetic support in the state of keeping durability of the non-magnetic support. From these viewpoints, there have been known so far base films for recording medium in which a variety of fillers are contained in the non-magnetic support to suppress influences of the surface state of the non-magnetic support (see JP-A-60-127523, JP-A-60-246919 and JP-A-2000-57558, etc.).
However, by only containing these fillers in the non-magnetic support, it is impossible to thoroughly suppress influences of the surface state of the non-magnetic support against the magnetic layer. In some case, characteristics of the non-magnetic support are largely changed. Accordingly, it could not be said that this means is effective from the viewpoint of keeping durability of the non-magnetic support.
On the other hand, there is also developed a magnetic recording medium in which an undercoat layer is provided between a non-magnetic support and a magnetic layer, or between a non-magnetic support and a non-magnetic layer, thereby suppressing influences of the surface state of the non-magnetic support against the magnetic layer (see JP-B-57-42890 and JP-B-60-38767, etc.).
However, though a polyester based resin is used for such an undercoat layer, such a construction involves a problem that rigidity of the medium is lowered so that durability is deteriorated.
Incidentally, the following JP-A-2004-39052 discloses a magnetic recording medium comprising a non-magnetic support having a smoothed layer and a magnetic layer containing a ferromagnetic powder and a binder in this order on at least one surface thereof, wherein the thickness of a filler to be contained in the non-magnetic support is from 0.5 to 5 nm, an aspect ratio of a mean particle size to the thickness of the filler is from 50 to 10,000, and the thickness of the smoothed layer is from 0.3 to 3 μm.
However, according to the construction as described in JP-A-2004-39052, while influences of the surface state of the non-magnetic support against the magnetic layer are suppressed due to contribution of the smoothed surface, there was involved a problem that rigidity of the medium is lowered so that durability is deteriorated.